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Chen et al. Microstructures 2023;3:2023025 https://dx.doi.org/10.20517/microstructures.2023.12 Page 17 of 31

TiN/G, CrN/G, WN/G, MoN/G, and binary-TiCrN/G, TiWN/G and TiMoN/G nanocomposites with a
[130]
large surface, high porosity, and excellent electrical conductivity (as shown in Figure 7I) . After loading
Pt NPs, the binary Pt/Ti Cr N/G catalysts exhibit the best ORR activity and stability due to the enhanced
0.5
0.5
exposure of active sites, the large accessible active sites, and the improved specific surface area and porosity,
resulting in strengthened electron transfer between Pt, Ti Cr N, and graphene.
0.5
0.5
From the above results, it is clear that the use of TMNs as support is an effective method to enhance the
ORR activity and stability of Pt-based catalysts. Firstly, the overall structural stability of the catalyst can be
enhanced by the SMSI between TMN and Pt. In addition, the inherent ORR performance and excellent
electrical conductivity of TMNs can further promote the catalytic activity in TMNs-supported Pt catalyst
systems. Moreover, the ORR catalytic activity can be further enhanced by doping TMN supports with other
elements due to the modulated structure and composition of the supported Pt nanomaterials. The detailed
ORR performance of Pt/TMN catalysts is presented in Table 3.

Carbides-based materials
Apart from TMO and TMN, TMC also possesses the potential as the ORR catalyst support because of their
chemical stability and high electrical conductivity. First, the similar electronic structure and catalytic
properties of TMC and Pt-group metals can reduce the overall loading of precious metals. Second, Pt-based
metals tend to bind firmly to metal-terminated TMC surfaces and facilitate the electron transfer with its
support to improve ORR stability and intrinsic activity [131-133] .

Titanium carbide (TiC)
Titanium carbide (TiC) possesses similar structural and physicochemical properties as TiN. This results in a
strong interaction between the TiC support and the Pt nanoclusters, which leads to an increase in the
adsorption strength of the oxygen molecules on the Pt surface and improves the ORR activity of the Pt/TiC
catalyst. According to the DFT calculations, the TiC support can strongly anchor Pt NPs and prevent their
exfoliation or agglomeration due to the formation of Pt-C-Ti bonds (as shown in Figure 8A) . Moreover,
[134]
Pt-C-Ti bonds were also demonstrated to possess stronger electronic interaction as compared to Pt-N-Ti
and Pt-O-Ti bonds, which leads to higher accessibility of active Pt sites in Pt/TiC and presents a better ORR
activity and stability than Pt/TiN and Pt/TiO catalysts . Lee et al. synthesized a high conductive two-
[71]
2
dimensional Ti C (MXene)-supported Pt NPs catalyst (Pt/Ti C ) and precisely tuned the number of layered
3 2
3 2
Ti C to provide more electron transfer for Pt NPs . DFT calculation showed that electron-rich Pt had
[135]
3
2
fewer d-band vacancies, weakening the binding of oxygen species to the Pt surface and increasing the rate of
*OH desorption. Furthermore, the electron transfer between Pt and Ti C triggers the SMSI effect, inducing
2
3
encapsulation phenomena of metal support and ensuring the stability of the Pt/Ti C catalysts.
3
2
Subsequently, Xie et al. presented a Pt/ Ti C X (X = OH, F) catalyst and found that the -OH and -F groups
3
2 2
can also prevent Pt NPs from agglomeration, Ostwald ripening, and dissolution, as does SMSI effect .
[136]
Nevertheless, the TiC substrate is not electrochemically stable and will undergo irreversible electrochemical
oxidation. To solve this problem, a Pt Pd/TiC@TiO core-shell composite catalyst has been reported to
2
3
possess better ORR stability than that of Pt/TiC and Pt Pd/TiC catalysts by Ignaszak et al. due to the TiO
3
2
[137]
shell can act as a protective layer over the TiC core . Besides the core-shell protection strategies,
electrochemical modification is another effective method to obtain oxygen-rich species on catalyst surfaces.
A surface aluminum-leached Ti AlC -supported Pt NPs catalyst (Pt/e-TAC) with much-improved ORR
3
2
activity and stability compared to Pt/C has been synthesized by Xie et al. (as shown in Figure 8B) . DFT
[138]
calculations indicated that the enhancement mechanism of Pt/e-TAC is attributed to the stronger
interaction that exists between Pt clusters and Ti C with respect to C. The partial density of states (PDOS)
13
3 2
confirmed that the SMSI effect between Pt and Ti C results in a considerable overlap between the Pt-d and
3
2
Ti-d states near the Fermi energy level.